1. Field of the Invention
This invention relates to an electrostatic atomizing device and a process thereof for the formation of electrostatically charged droplets having an average diameter of less than about 1 millimeter for a liquid having a low conductivity, wherein the device includes a cell having a chamber disposed therein, a discharge spray means in communication with the cell, the liquid in the chamber being transported to the discharge spray means and atomized into droplets, and a mechanism for passing a charge through the liquid within the chamber, wherein the charge is sufficient to generate free excess charge in the liquid within the chamber.
2. Description of the Prior Art
The literature is saturated with various types of electrostatic atomizing devices which are of limited adaptability due to a number of factors such as the inability to be functionally operable in air, the inability to atomize low conductivity liquids, and the inability to form droplets having an average diameter of less than about 10 microns with commercially acceptable flow rates.
U.S. Pat. No. 3,358,731 is a combustion burner device having a diode type electrostatic atomizing device, wherein the charged droplets are attracted to a downstream charged surface having a lower electrical potential than the atomizing device. This atomizing device produces droplets having large diameters.
U.S. Pat. No. 3,597,668 relates to an electrostatic fuel charging device for use with an internal combustion engine, wherein a friction element disposed within a cylindrically shaped casing imparts an electrostatic charge to a liquid fuel flowing through the casing.
U.S. Pat. No. 3,167,109 relates to a diode type electrostatic atomizing device employing a convective flow of charged air in order to electrostatically charge a liquid externally to a liquid supply cell.
The device of U.S. Pat. No. 2,525,347 is a spray diode. In all instances cited electrostatic atomization of fluid occurs from a small radius of curvature edge or edges over which the fluid passes. Atomization proceeds in response to an electric field established between this edge and the object or objects toward which the spray is to be directed. High potential voltage is stated as being necessary for operation in conjunction with the object, nozzle separation distances noted. Without exception operation is at ambient conditions with the interelectrode gap in air.
This invention differs from the process claimed here in that no claim is made with respect to forceable charge injection into the spray fluid. Since the spray fluid is charged by charge release from a sharpened surface in response to large electric fields developed by large potential differences operating over large air gaps, the flow rate is restrained to that capable of just forming a thin layer over the sharp edge. This is consistent with the absence of the third electrode whose presence within the liquid would assure sufficient charge injection to permit large volume flow rate spraying.
U.S. Pat. No. 3,775,193 teaches that a passivating liquid flows thru an aperture through which an electrode protrudes. A high intensity electric discharge is maintained between this electrode and the metallic surface being passivated by the fluid. This operation is typically conducted in a vacuum below 10.sup.-4 torr. The discharge between the electrode and surface produce atomization of sufficient intensity to reduce droplet size below 200 .mu.m.
Small flows are indicated as being associated with this procedure. The presence of only two electrodes, the central spray head electrode and the work surface distinguish this invention from the spray head concept of the instant invention. The of high intensity electric discharge between the head electrode and the work piece noted as being central to the methods' mode of operation and to the atomization process indicates the principle of operation is functionally distinct from the charge injection process of the spray diode.
U.S. Pat. No. 3,167,109 reveals a coaxial device in which electrostatic fields are used to: (1) provide an "electric wind" effect to move air to the combustion zone, and (2) produce atomization and spraying of liquid fuel into the air preparatory to combustion. Electrostatic atomization of fuel occurs from a centrally located supply electrode in response to a potential diference that exists between it and another electrode. Air moves within the annular space defined by the two electrodes.
Electrostatic atomization is limited in this device by the maximum electric field capable of being sustained across the air gap. This device is clearly a spray diode since it lacks an essential electrode immersed in the spray fluid. As a consequence its spray performance is limited to low flow rates.
U.S. Pat. No. 3,269,446 teaches that the liquid fuel is supplied to an annular manifold from which it flows vertically downward and radially outwardly over a conical surface, the lower edge of which is sharp. Electrostatic spraying occurs from this edge by action of an electrostatic field that exists between the spray cone and an annular coaxially positioned ring electrode having a larger diameter, and placed lower than the cone. An alternate embodiment of this device replaces the second ring electrode by a circular cylindrical screen electrode surrounding the centrally located spray cone. A second alternation replaces the cone with a horizontal sharp disc having a sharp edge from which spraying occurs to the ring electrode.
In all three instances an air gap exists between the two (and only) electrodes responsible for spraying. Consequently the devices are all subject to air breakdown effects which limit their capability to spray large volumes of low conductivity liquids--the devices are spray diodes--being only a pair of electrodes.
A paper by Tsui and Hendricks (RSI, Vol. 39, Aug. 1969) reveals a coaxial device designed to disrupt an otherwise uniform column of liquid into a co-linear stream of uniform sized droplets (.about.300 .mu.m diameter). This is accomplished by positioning a pointed rod coaxially with the exit hole through which the liquid flows. Imposition of an alternating voltage differential between the pointed electrode and the orifice plate produces the desired disruption, but only in a well defined frequency range.
The alternating voltage is used solely as an oscillating electrohydrodynamical pressure source. It is this periodically varied pressure that produces the desired breakup. This device does not suggest a means for developing spray clouds of small droplets as in the Spray Triode. The Tsui/Hendricks paper, therefore, is nonapplicable to the Spray Triode of the instant application.
Paint spraying at elevated voltages (70 KV to 100 KV), as seen in U.S. Pat. No. 3,512,502, is produced by rotating a sharp edged truncated cone maintained at high potential with respect to the grounded object to be coated. Paint is fed to the inside of the spray cone and is atomized as it leaves the sharp forward lip. Atomization proceeds by a combination of centrifugal and electrostatic forces.
A third electrode in the form of a small pointed cone is centrally located, i.e. is coaxially positioned and is approximately co-planar with the spray lip. A resistor is used to maintain this tip at a potential intermediate with respect to the spray cone and the grounded target. The device is a true Triode, the first thus far identified as prior art. However, a clear distinction between this device and the Spray Triode can be made insofar as the central third electrode is expressly used to control spray pattern geometry by altering the electrostatic field in the vicinity of the spray lip. Moreover, the central electrode is separated by an air gap from the spray lip. The conical third electrode does not contact the spray liquid directly as is the case in the instant application and contact is noted as to be avoided for correct operation.
The spray coating apparatus as seen in U.S. Pat. No. 3,700,168 is a coaxial device. Grounded spray liquid is radially flowed outward from a central supply tube toward a concentrically positioned electrode. High voltage is supplied to this electrode via a current limiting resistor. An air flow is maintained in the annular space between the inner liquid supply tube and the outer electrode support tube. The air flow, normal to the radially directed liquid flow, produces atomization and prevents collection of liquid on the high voltage electrode. It is stressed that collection of liquid on this electrode is deleterious to proper operation. This is clearly a diode, since the target is also at ground potential.
However, a coaxial, electrically floating cylinder has also been included in the description. It is the purpose of this cylinder to provide an electric field component to force the spray droplets out of the spray head which is encased in a cylindrical grounded enclosure. The patent proceeds to elaborate on this "driving" electrode and described a unique design that can be added to the gun to improve its spray pattern.
In place of the electrically floating cylinder a "driving" electrode charged to high potential by air-ion collection is detailed. The unique feature of this concept resides in the use of feed air stream kinetic energy to forcibly convert air-ions to the "driving" electrode. The kinetic energy of the air stream overcomes the retarding field of the "driving" electrode permitting high potentials to be attained at modest operating voltages. The "driving" electrodes is, therefore, charged by the equivalent of an air driven Van de Graaf generator.
Again the use of supplemental means for atomization, lack of emission from the electrode, and the absence of direct, forcible charge injection clearly distinguishes this device from the Spray Triode of the instant application.
A spray coating apparatus as seen in U.S. Pat. No. 3,587,967 is a Spray Diode. In addition, air is used to augment the atomization process. This device has coaxial geometry and uses a centrally positioned, sharply pointed high voltage electrode. Since the electrode is in air or in an air, droplet mixture, its function is not similar to the Spray Triode emitting electrode. therefore it cannot be cited as prior art.
The spray charging device as seen in U.S. Pat. No. 3,698,635 is a spray diode by virtue of the fact that two of the three electrodes used are at the same potential. In particular, the liquid feed tube and the target are both grounded. Liquid is fed through the innermost of three coaxial tubes. This feed tube is a dielectric in which a grounded electrode makes contact with the conductive spray fluid upstream from the liquid exit position. The liquid is forced radially outward from the end of the tubes. An enclosing concentric tube, also of dielectric material, supports a high voltage electrode coaxially in the vicinity of the liquid exit slot. This electrode is connected via a current limiting resistor to a high voltage supply. As liquid exits the inner tube, it is atomized partially by action of the electrostatic field produced by the high voltage electrode on the conducting spray fluid. Atomization is augmented by a high volumetric flow rate of air in the annular space defined by the two tubes. Liquid resistivities as high as 1.3.times.10.sup.4 .m are quoted as being sprayed by this device, a claim is made for 1.5.times.10.sup.5 .m as the maximum resistivity level. The air flow is noted as being 10.sup.3 times that of the liquid. This high flow rate assures atomization and prevents liquid from accumulating on the high voltage electrode. Liquid contact with this electrode is noted as being inimical to optimal performance. The operation of this device is at 4 to 7 KV with an annular gap spacing of about 1/2 mm. The entire unit is enclosed in an open-ended grounded metallic cone. A second version of this device is also described. In this version liquid is coaxially flowed out of a 1.52 mm ID nozzle on the centerline. The end of this tubular nozzle is coaxial with a high voltage electrode and separated from it by an annular gap of .about.0.9 mm through which air is forced. The indicated liquid flow rates were 0.83 to 4.67 ml/Sec with air flow again about three orders of magnitude higher (1420 ml/Sec). Indicated mean charge to mass ratios of 4.2.times.10.sup.-3 C/kg at 0.83 ml/Sec and 2.0.times.10.sup.-3 C/kg at 4.67 ml/Sec for this device place it in precisely the same performance category as the present apparatus. It should be noted that the instant invention attains the same charge levels but with a fluid some 10.sup.9 times more resistive and without need of an air flow. This spray unit is non-applicable to our patent application. It is noted that a third electrode can be added coaxially with the device and at its exit. It is the purpose of this electrode to help shape the spray geometry, i.e. to concentrate it in the forward direction. With this electrode in place, the unit is a spray triode but of the same type as represented in U.S. Pat. No. 3,512,502.